#include <stdio.h>
int main()
{
	double sum = 0, term;
	int i;
	for(i=0; ; i++)
	{
		term = 1.0 / (i*2+1);
		if(i %2 == 0)	sum += term;
				else	sum -= term;
		if(term < 1e-6)	break;
	}
	printf("%.6f\n", sum*4);
	return 0;
}
